So I'm working on an Alphonse Elric cosplay (full armor) and I'm not sure what color to paint it. In the anime and manga both, it never seems terrible shiny but it does seem to be made of metal. My question is this: Should I paint my armor a flat (matte) grey- or even a blueish grey- or a metallic silver?

Sorry if I'm not giving credit to these cosplayers correctly but here's a few examples of what others have done, just tell me which ones you guys like best!

The reason it doesn't appear shiny in manga or anime is because shiny doesn't translate well to print or animation source. It's easier and faster to go with a flat color.

I like the bluish grey, but Viveeh is right about the metal. The matte colour doesn't translate to a metallic surface very well. Would there be some way to incorporate both the blue-grey and a metallic look, that way, you'd have the best of both?

Yeah, depending on how you wanted it to look, you could potentially underpaint with blue-grey and then do a thinned coat of metallic.
Another option is to do a blue-grey with a gloss coat to get some shine, or alternatively you could do it metalic blue and then paint it over with a matte varnish, this would then kill a lot of the shine, but leave some of the metalic feel.

I would say if you have access to an airbrush, make the best of both worlds. Mix a blue in with silver with a good 10-80 mix and give a few test runs. Mix and match untill you find exactly what you like, and then expand on it.

I know Alclad has some amazing paints, but cost/amount ratio would be a killer.

Also check under titanium labeled paints, as they often have a blue hue to it, you might find one you really like.

I'd go with the Blue color....it looks more true to the anime...however, i have figures of Alphonse that give him a silver color. very badass looking. all in all it depends on what you think would look best.